The present invention relates to electrical measuring equipment and, in particular, to a novel kilovoltmeter which is especially useful for measuring the kilovolt output of the secondary winding of an ignition coil.
The task of accurately measuring the kilovolt output of an ignition coil secondary winding is not an easy one. Presently available equipment for measuring the kilovolt output of an ignition coil is often bulky, difficult to transport, relatively complicated, hard to set up and adjust, and may not be universally suited for the various possible types of ignition coils and systems currently in use. Further, the accuracy of such equipment may be less than optimum where the secondary voltage is characterized by a very fast rise time, as in a capacitor-discharge ignition system. The present invention is directed to providing a novel kilovoltmeter which is generally less complicated than prior kilovoltmeters yet which is capable of providing an accurate measurement of ignition coil secondary voltage, and without complicated setup procedures. Furthermore, the invention provides a compact kilovoltmeter which is conveniently transportable and which can be readily connected and set up with an ignition coil whose secondary voltage is to be measured. The invention may be used with virtually all ignition systems and coil types and is especially good for accurately measuring the secondry voltage of a capacitor-discharge system which is characterized by a very fast rise time. Moreover, this measurement can be made regardless of the polarity of the secondary voltage rise, or the specific shape of the secondary voltage waveform. Further, the invention permits separate measurements of the positive and negative polarity portions of the unloaded secondary voltage waveform with only a single galvanometer being used to provide a reading of both measurements.
A kilovoltmeter according to the present invention comprises an electronic circuit which is coupled with an ignition coil secondary and which drives a galvanometer in accordance with the secondary voltage to provide a reading of the secondary voltage magnitude. One feature includes an operational amplifier which amplifies an input signal representative of the ignition coil secondary voltage to provide an output signal to the galvanometer. Another feature of the invention resides in a novel input circuit which is used to connect the secondary of the ignition coil with the input of the operational amplifier. This input circuit includes a probe assembly which plugs into a console containing the kilovoltmeter circuitry. This probe assembly includes a capacitive probe element used to make contact with the hot terminal of the secondary winding and a length of high-tension, high-resistance conductor which leads from the probe element to the console. The probe element and conductor connect in series with each other and between the secondary and the operational amplifier. The kilovoltmeter circuitry also has a capacitor operatively connected with the probe assembly to cooperate with the capacitive probe element and the high-resistance conductor to provide a voltage-dividing circuit which supplies to the operational amplifier a voltage-divided signal representative of the actual secondary output voltage, but appreciably attenuated. This voltage-divided signal is passed through additional filter and attenuating stages to develop the input signal which is applied to the input of the operational amplifier. The input circuit also has a switch means to select either positive or negative polarity portions of the secondary output for waveform measurement. A novel zeroing adjustment is also provided to zero the galvanometer needle, and this is derived as a selectable reference voltage from a reference circuit supplied as a reference input to the operational amplifier. A further feature resides in the output circuit which connects between the output of the operational amplifier and the galvanometer. The output circuit includes a switch means which permits both positive and negative output signals of the amplifier, which are representative of negative and positive secondary voltage, to be measured by the galvanometer even though the galvanometer itself is operable from only one polarity signal. Included in the output circuit are two calibrating resistors, each of which calibrates the output of one polarity of the amplifier output signal.
Additional features and advantages of the invention, along with those already enumerated, will be seen in the ensuing description and claims which are to be takin in conjunction with the accompanying drawing.